The invention in general relates to interferometers and, more particularly, to geometrically-desensitized interferometer (GDI) instruments that utilize nondiffractive optics for full-field measurement of the profiles of surfaces.
Instruments for profiling surfaces are generally classified as either contact or noncontact types. With contact types, a stylus is used to mechanically move over the surface while in physical contact with it to build up information about surface features including their position and scale. Noncontact types are usually optically based and may be either scanning types or full-field types depending on whether or not a probe is moved over a surface in the manner of a stylus but not in contact with the surface or an area larger than that measured by a probe is imaged all at once.
Optical metrology of surface profiles can generally be divided into two regimes, namely interferometric and geometric. Geometric techniques include triangulation and moire fringe analysis, which involves the projection and imaging of a periodic structure such as a Ronchi ruling. Geometric techniques are relatively insensitive to surface roughness and deformations, but are of relatively low resolution thus making them unsuitable for many applications in which surface profiles must be measured with high precision.
Interferometry, on the other hand, relies on the wave nature of light for high precision measurement of the surface profile of a test object. A typical interferometer includes a light generator that produces a coherent beam of light followed by a beam divider that splits the beam into reference and measurement beams. The reference beam is then reflected off a reference surface, and the measurement beam off the object whose surface is to be profiled. First and second reflected wavefronts from the reference and measurement surfaces are then recombined with one another while interfering both constructively and destructively to produce an interference fringe pattern, the fringe pattern being a function of the optical path difference between the paths traveled by the reference and measurement beams. The optical path difference results in differences in phase as a result of the differences in optical path traveled between the reference and measurement beams. An imaging device, such as a solid state camera, receives the recombined wavefronts and acquires images of the interference fringe pattern. The interference fringe pattern then is analyzed to obtain information about the surface profile of the test object.
Fringe pattern analysis for surface profilometry often is performed by the well-known technique of phase shifting interferometry (PSI). In PSI, the height difference between locations on a surface imaged by first and second pixels on the imaging device is determined by first determining a phase difference between light received at the first and second pixels and by then using the phase difference to calculate a height difference. A primary advantage of PSI is that it is highly precise. The vertical height precision for PSI is a fraction (e.g., 1/100) of the optical wavelength of the light source used to conduct the measurement. A second advantage of PSI is that it has good vibration immunity characteristics because phase data is acquired for all pixels simultaneously and because the data acquisition time is relatively short.
Generally speaking, however, conventional PSI approaches can profile only smooth surfaces having relatively small height variations or "surface departures" between adjacent measurement sites since conventional interferometry on a surface with high slopes generates such a high fringe density that no meaningful information can be derived from the fringe pattern. Therefore, while PSI interferometry is much more precise than geometric optical profilometry, it historically has been considered to be ill-suited for use with rough objects or objects having marked surface deformations.
One interferometric technique that lacks the quarter-wavelength constraint of PSI is the so-called scanning white light interferometry or SWLI. In SWLI, a white light illumination source or, more generally, one which is of a broad-band spectrum as opposed to being of a narrow-band spectrum (for example a laser), generates an interference pattern which contains, as a function of scan position, regions of high contrast for each location on the test surface. The scan position of high contrast for a given pixel indicates the height of the corresponding location on the test surface. Therefore, by comparing the temporal characteristics of these regions of high contrast with one another, a difference in height between two locations on the profiled surface can be determined. Unlike PSI, SWLI does not calculate height differences based on phase differences, and the PSI phase constraint therefore does not apply to SWLI. The maximum physical departure between adjacent measurement sites on a profiled surface therefore may be much larger with SWLI than with PSI.
However, SWLI has disadvantages of its own that hinder its use in industrial applications. For instance, the field of view is generally no larger than can be accommodated by standard microscope objectives.
Another disadvantage of typical SWLI techniques is that data acquisition is very slow. The slow speed is a consequence of the large amount of data which must be acquired, given the rate at which the interference effect varies with scan position. The slow speed creates additional problems, such as a high sensitivity to thermal distortions and mechanical strain during measurement.
Still another disadvantage of typical SWLI is its high sensitivity to vibration, which is due in part to the slow data acquisition speed, and in part to the extremely high sensitivity of the interference fringe pattern, which is easily corrupted by very small amounts of vibration. Recent years have seen an increased demand for the high speed, high precision metrology of the surface profiles of manufactured parts having large surface departures, i.e., having rough surfaces or surfaces with pronounced surface deformations. A corresponding demand has arisen for the acquisition of data during production as opposed to in the laboratory. For instance, precision products such as hard disks for computer disk drives need to be profiled with high precision, at high speeds, and under conditions in which the test object may be subjected to substantial vibrations during manufacturing processes. Neither traditional PSI techniques nor traditional SWLI techniques are suitable for these purposes. A need therefore has developed for a "desensitized" interferometer that is relatively insensitive to surface roughness and surface deformations, that performs surface metrology with high accuracy and at high speeds, and that is relatively insensitive to vibrations and therefore is well-suited to production-line use.
This need has been met to a large extent by the development of the geometrically-desensitized interferometer (GDI) instrument. A GDI instrument is characterized by the replacement of the beam splitter of the traditional instrument with an optical assembly located between the collimating lens and the test object. The optical assembly divides the collimated light source into two beams which propagate in two different directions and impinge on the profiled surface at the same location but at different incident angles. The beams reflect from the profiled surface and pass back through the optical assembly in different directions, after which they are recombined. Constructive and destructive interference of the reflected and recombined beams form an interference fringe pattern having an equivalent wavelength A that may be orders of magnitude larger than the source wavelength. As a result, GDI instruments are much less sensitive to height variations and surface deformations than are traditional interferometers using PSI analysis techniques. The sensitivity of GDI instruments is between that of conventional interferometry and moire fringe analysis, and is comparable to that obtained with grazing-incidence interferometry. GDI instruments therefore can be used in manufacturing applications and other applications that are unsuitable for traditional interferometry.
An exemplary GDI instrument is disclosed in U.S. Pat. No. 5,526,116 to de Groot (the de Groot '116 patent). Specifically, FIG. 2 of the de Groot '116 patent illustrates a diffractive optical assembly that includes first and second parallel linear phase gratings spaced from one another in the Z-direction of the instrument. The second grating produces the advantage of permitting the working distance between the exit surface of the grating assembly and the profiled surface of the test object to be typically about 2 inches. Both gratings are involved in both the splitting of an inbound beam and in the recombining of reflected beams from the object surface. Specifically, the first grating diffracts an inbound collimated beam from a light source into two first-order beams "A" and "B". The beams A and B are then redirected by the second grating so that they impinge on the profiled surface of the object at the same location but at different incident angles, .alpha. and .beta.. Reflected beams A' and B' propagate outwardly from the profiled surface at corresponding angles .alpha.' and .beta.' and travel sequentially back through the second and first gratings to recombine with constructive and destructive interference. The recombined interfering beams, or wavefronts, form an interference pattern that is imaged onto a photodetector that generates a signal from which the pattern may be displayed and the surface profile determined. This information may also be converted to 3D visual map of the surface topography. Typically, the interference pattern provides for each point on the imaged object surface an interference phase that is substantially linearly proportional to the local surface height.
The typical grating-based GDI instrument exhibits all of the above-described advantages of GDI instruments. Moreover, because it requires only two optical elements to split and recombine the inbound and outbound beams, it is relatively compact, relatively easy to align, its alignment is relatively easy to maintain, and it has a relatively small sensitivity to air turbulence. Moreover, it constitutes a true full-field GDI instrument., The basic geometry of a full-field instrument provides a substantially linear and uniform response to surface topography over the entire imaged area of the object surface.
Some types of prior-art GDI instruments rely on beam splitters, lenses, mirrors, and/or other non-diffractive optical elements to split and recombine beams. GDI instruments of this type are disclosed, e.g., in U.S. Pat. No. 3,958,884 to Smith (the Smith patent), U.S. Pat. No. 4,714,348 to Makosch (the Makosch patent), and U.S. Pat. No. 5,568,256 to Korner, et al. For instance, the instrument illustrated in FIG. 6 of the Makosch patent employs a beam splitter and a plurality of mirrors to generate three collimated beams that are brought to interference in a symmetrical light field. When an object surface is brought into this light field, reflected beams are transmitted back through the system of splitters and mirrors and are recombined to form an interference pattern representative of the profile of the imaged surface.
However, known non-diffractive GDI instruments apparently have at least one very serious limitation that severely limits their practical range of applications. That is, unlike grating-based instruments, known conventional optics-based GDI instruments do not have full-field capability. This limitation arises from the fact that the field-independent path difference condition of a full-field instrument is not fulfilled by the mirrors or other conventional optical elements typically used in these instruments. For example, in the Makosch patent, the inbound beams exhibit a first OPD at one field position on the object surface as illustrated in FIG. 6 and exhibit a different OPD at a different field position, even if the object surface is perfectly flat. Depending on the manner in which interference data is acquired and interpreted, the actual field of view of the typical non-diffractive GDI instrument may therefore constitute no more than a point on the object surface relative to the instrument or, at best, a line extending along the object surface. Therefore, conventional optics-based instruments generally are limited to single-point profiling or, at best, linear profiling. A need therefore has arisen to provide a GDI instrument that has full-field imaging capability but does not necessarily require diffractive optic components, and it is a primary object of the present invention to provide such an instrument.
Other objects of the invention will in part be obvious and will in part appear hereinafter when reading the detailed description in connection with the various drawings.